Due to granule size, substrate and oxygen become limited in the core of granules leading to cell lysis at the core. Loss of granule stability is still a major barrier for practical application of AG. The higher the DO, the deeper the oxygen penetration inside AG. According to Henry's Law, DO increases with increasing oxygen pressure in gas phase. Compared to ambient pressure condition (AP), operation of AG under high pressure (HP) might a favorable condition for formation and stability of granules.
Experimental results show that granulation was facilitated under HP condition. MLSS and size of granules under AP system are higher than those under HP system. However, SS of effluent in AP is higher than those in HP and is consisted mainly with flocculent sludge. Longer SRT and lower biomass yield are obtained in HP system, indicating that less sludge will be produced in HP system.
HP system can operate at high nitrogen loading. Complete nitrification was observed earlier in HP, indicating that the growth of NOB was facilitated under high dissolved oxygen. The (SOUR)h,(SOUR)NH4 and (SOUR)NO2 of HP sludge are better than of AP sludge. However, HP sludge is more sensitive to changes of FA and FNA concentrations due to smaller granule size.